1. Field of the Invention
The present invention relates to a liquid crystal display device for use in display apparatuses of computer terminals, television receivers, word-processors, typewriters and the like, optical bulbs of projectors, view finders of video cameras, and the like.
2. Description of the Related Art
Various liquid crystal (LC) display devices are known, for example, twisted nematic (TN) LC display devices employing twisted nematic liquid crystals, guest-host LC display devices, and smectic (Sm) LC display devices employing smectic liquid crystals.
In such LC display devices, the liquid crystal sandwiched between substrates changes its transmittance according to the applied voltage. Thus, the strength of the electric field applied thereto varies depending on the size of the inter-substrate gap, that is, the thickness of the liquid crystal layer.
Clark and Lagerwall disclosed a bistable ferroelectric liquid crystal (LC) device employing a surface-stabilized ferroelectric liquid crystal in, for example, Applied Physics Letters, vol. 36, 11 (Jun. 1, 1980), pp. 899-901, Japanese Patent Application Laid-open No. 56-107216, U.S. Pat. Nos. 4,367,924 and 4,563,059. In this bistable ferroelectric LC device, the liquid crystal is disposed in a gap between a pair of substrates arranged so that the gap size is sufficiently small to inhibit the formation of helical structure of the liquid crystal molecules which usually occurs in chiral smectic C phase (SmC) and H phase (SmH) in bulks of the liquid crystals, and homeotropic molecule layers formed of a plurality of LC molecules are aligned in a single direction.
In addition, display devices employing such ferroelectric liquid crystals (FLCs) are also described in U.S. Pat. Nos. 4,639,089, 4,655,561 and 4,681,404. In the display devices, an FLC is filled in a gap of a liquid crystal cell comprising two glass substrates spaced apart from each other by about 1-3 .mu.m, the inside surfaces thereof having been provide with transparent electrodes and alignment-treated.
The above described FLC display devices have the following advantageous features. Because FLCs have spontaneous polarizability, the binding force of spontaneous polarization can be utilized together with an external electric field for switching. Because the directions of the long axes of FLC molecules and the directions of spontaneous polarization show one-to-one correspondence, the FLC display devices can be switched according to the polarity of an external electric field. More specifically, because FLCs in the chiral smectic phase exhibits bistability, that is, they quickly assume one of the first and second optically stable states in response to application of an electric field, and remains in the same optically stable state even after the electric field is discontinued, FLC display devices are expected to be widely employed in various fields, for example, high-speed and memory-type display apparatuses.
As described above, although chiral smectic liquid crystals (SmC, SmH), widely-used FLCs, exhibit in bulk alignments in which the long axes of the liquid crystal molecules are twisted, the twisting of the long axes of liquid crystal molecules is eliminated if such a liquid crystal is filled in a cell gap having a size of about 1-3 .mu.m (N. A. Clark et al, MCLC (1983) vol. 94, pp. 213-234).
A liquid crystal display apparatus comprising an FLC as described above can employ a display panel comprising large-capacity pixels to display images if it uses the multiplexing drive method disclosed by Kanbe et al in, for example, U.S. Pat. No. 4,655,561. Such LC display apparatuses can be used in word processor, personal computers, microprinters, televisions.
Although FLC devices generally use the two stable states to transmit and block light, that is, to perform binary (black and white) display, they can also perform multivalued (gray tone) display. A gray tone display method, called area modulation method, achieves intermediate light transmitting states by controlling the ratio between the areas in the bistable states in a pixel. The area modulation method will be described below.
FIG. 1 is a graph indicating the relation between the transmittance and the amplitude V of a switching pulse in an FLC device. The horizontal axis of the graph is the logarithms of the amplitude V of a single pulse having one polarity, and the vertical axis is the amount I of light transmitted through a cell (device) while the cell changes from the light blocking (black) state to the light transmitting (white) state by application of the single pulse thereto. When the amplitude of the pulse V is less than a threshold V.sub.th (V&lt;V.sub.th), the amount of transmitted light does not increase, that is, the state of the pixel exhibits no change, as shown in FIGS. 2(a) and 2(b) indicating the states before application of a single pulse and the state immediately after the pulse application, respectively. When the pulse amplitude exceeds the threshold (V.sub.th &lt;V&lt;V.sub.sat), part of the pixel changes from the light blocking state to the light transmitting state, as indicated in FIG. 2(c). Thus, the pixel as a whole transmits intermediate amounts of light. When the pulse amplitude becomes greater than the saturated value V.sub.sat (V.sub.sat &lt;V), the entire pixel assumes the light transmitting state as indicated in FIG. 2(d). Thus, the amount of transmission reaches a constant value. The area modulation method achieves gray tones by controlling the applied voltage so that the pulse amplitude V becomes a value within the range V.sub.th &lt;V&lt;V.sub.sat.
However, because the amount of light transmission varies depending on the cell thickness and the temperature as well as the applied voltage, such a simple driving method as the area modulation method has a problem in that if the thickness or temperature differs from one location to another in a cell, the tone level of one pixel will be different from that of another pixel even when pulses of the same amplitude (voltage) are applied thereto.
The above-stated problem will be explained with reference to the graph of FIG. 3, similar to FIG. 1, indicating the relation between the amplitude (voltage) V of an applied pulse and the amount I of transmitted light. The lines H and L of the graph indicate the V-I relation at high and low temperatures, respectively. As indicated by the graph, an amplitude V.sub.ap of a single pulse gives gray tones varying within the range between I.sub.1 and I.sub.2 depending on temperature, thus failing to display a uniformly-toned image. Because such a non-uniform temperature distribution in a portion corresponding to a single pulse (a displaying portion) normally occurs, particularly, in a large-size display device, the area modulation method often suffers from this problem.
To eliminate the problem, the present inventors have proposed "4-pulse method" in U.S. patent application No. 681,993 filed on Apr. 8, 1991 (see FIGS. 4 and 5). This driving method applies a plurality of pulses pulses, (A), (B), (C) and (D) as shown in FIGS. 4 and 5) to low, intermediate and high-threshold portions in a single scanning line and thereby achieves substantially the same inverted areas in all the portions when the last pulse, pulse (D) in the aforementioned figures, has been applied.
The present inventors also proposes, in the specification of U.S. patent application No. 984,694 filed on Dec. 2, 1992, "pixel shift method" which requires a shorter write-in time than the 4-pulse method. The pixel shift method simultaneously inputs different scanning signals to a plurality of scanning signal lines and thus selectively obtain a distribution of electric field strength over the scanning lines in order to achieve gray tone display.
The pixel shift method will be briefly described. The LC cell used by the pixel shift method has threshold gradient in each pixel. FIG. 6 shows an example of such an LC cell. As shown in FIG. 6, because the thickness of an FLC layer 55 between the electrodes varies over each pixel, the switching threshold of the FLC accordingly varies over each pixel. If the voltage applied to such pixels is gradually increased, switching (state inversion) gradually occurs from portions having a small FLC thickness to portions having a large FLC thickness.
FIG. 7(a) is a graph indicating that the relations between the applied voltage and transmittance at three different temperatures, in which T1, T2 and T3 are the temperatures of a portion of the panel that is observed. As indicated by the graph, the switching threshold of the FLC decreases as temperature increases.
Although factors other than temperature can also cause the threshold to vary, the following description will be made only in connection with variation in temperature, to simplify the description.
As indicated by FIG. 7(a), when a voltage V.sub.i is applied at a temperature T.sub.1 to a pixel that has been entirely reset to the light-blocking state, the pixel achieves a transmittance of X %. However, if the temperature increases to T.sub.2 or T.sub.3, the application of the voltage V.sub.i to the pixel will provide a transmittance of 100%, thus failing to achieve a proper gray tone. FIG. 7(c) illustrates the status of the black-white inversion in a pixel at the temperatures T.sub.1, T.sub.2 and T.sub.3 after tone data have been written thereinto. Because the effect of tone data significantly varies depending on temperature as shown in FIG. 7(c), use of an LC device is limited to a significantly narrow temperature range.
However, stable tone display can be achieved despite temperature variation by employing the pixel shift method in which, as shown in FIG. 7(d), data for a pixel is shared by two adjacent pixels on different scanning signal lines S1 and S2.
This driving method will be described below in connection with three major features.
(1) The method uses an FLC cell having a continuously -varying threshold distribution in each pixel, for example: an FLC cell as shown in FIG. 6 in which each pixel has a continuously varying cell-thickness distribution; an FLC cell, as proposed by the present applicant in Japanese Patent Application Laid-open No. 63-186215, in which each pixel has potential gradient; or an FLC cell in which each pixel has capacity gradient. Because each pixel has a continuously varying threshold distribution, it can simultaneously have a light domain corresponding to the light transmitting state and a dark domain corresponding to the light blocking state. Thus, tone display can be achieved by controlling the area ratio of the light and dark domains.
This driving method can be employed to achieve stepwise tone display and continuous (analog) tone display. To achieve analog tone display, the amount of light transmitted through each pixel must be continuously varied.
(2) The pixel shift method simultaneously selects two scanning signal lines. This feature will be described with reference to FIGS. 8(a) and 8(b). FIG. 8(a) is a graph indicating the transmittance-voltage characteristic when two adjacent pixels on neighboring scanning signal lines are used in combination. In the graph, the transmittance range of 0-100% is assigned to indicate a display domain of a pixel B on a scanning line 2, and the transmittance range of 100-200% is assigned to indicate a display domain of a pixel A on a scanning line 1. In short, the transmittance 200% means that two adjacent pixels A and B have entirely assumed the light. transmitting state when the two adjacent scanning signal lines 1 and 2 are scanned. According to this method, two scanning signal lines are simultaneously selected in response to a piece of tone data, and a domain as large as one pixel is assigned for the piece of data, as illustrated in FIG. 8(b).
At temperature T1, tone data is written in as follows. The applied voltage V.sub.0 inverts an area corresponding to transmittance 0%, and the applied voltage V.sub.100 inverts an area corresponding to transmittance 100% thereof. In other words, at temperature T.sub.1, the area which undergoes stable state inversion when receiving voltage ranging from V.sub.0 to V.sub.100 (hereinafter, referred to as "the effective pixel area") coincides with the area consisting of the pixels B on the scanning signal line 2, as indicated by the shadowed area in FIG. 8(b).
If the temperature rises from T.sub.1 to, for example, T.sub.2, the threshold voltage of the liquid crystal accordingly decreases and, therefore, an area inverted (or a light transmitting domain achieved) by the same level of voltage will increase. To correct such a temperature-dependent increase of the inverted area, presetting is made so that the effective pixel area is shifted so as to lie over the pixels A and B on the scanning lines 1 and 2 without a substantial change in size, as indicated by the shadowed area associated with "T.sub.2 .degree.C." in FIG. 8(b).
If the temperature further rises to T.sub.3, the effective pixel area is shifted so as to coincide with the area consisting of the pixels A on the scanning signal line 1, as indicated by the shadowed area associated with "T.sub.3 .degree.C." in FIG. 8(b).
(3) The pixel shift method applies different scanning signals to the two scanning lines simultaneously selected. To compensate for temperature-dependent variation in the threshold voltage for LC state switching by simultaneously selecting two scanning signal lines, the scanning signals applied to the two scanning signal lines must be different from each other. This feature will be described with reference to FIGS. 7(a) to 7(d).
The scanning signals applied to the scanning lines 1 and 2 are formed so that the switching threshold continuously varies from the pixels B on the scanning signal line 2 to the pixels A on the scanning signal line 1, that is, as shown in the graph of FIG. 7(b), a transmittance-voltage line is linear and continuous from the pixels A to the pixels B. Like the graph of FIG. 8(a), the graph of FIG. 7(a) indicates that it the transmittance is within the range of 0-100%, only the pixels B on the scanning signal line 2 are used for display, and that, if it is within the range of 100-200% the pixels A on the scanning signal line 1 are also used for display.
Thereby, even if the pixels A and the pixels B have the same cell shape as illustrated in FIG. 9(b), they can perform tone display comparable to the tone display performed by a cell, as shown in FIG. 7(b), actually having a switching threshold gradient substantially continuous from the pixels B to the pixels A.
If the pixel shift method is employed to perform tone display, it is preferable that the erasing (reset) orientation be switched every scanning line or every frame. However, because the cycle of reset to the light transmitting state (white reset) and reset to the light blocking state (black reset) causes a light quantity change during the resetting period or after the write-in period, a viewer may perceive flickering of the display.
In addition to the above problem, there is another problem. Even when the same tone level needs to be maintained, the tone level after white reset and the tone level after black reset differ from each other due to light leaking during reset periods or after write-in periods.